Polyfill模拟出ES5++环境,所以会包含在编译后的源代码中

Polyfill

🚨 As of Babel 7.4.0, this package has been deprecated in favor of directly including core-js/stable (to polyfill ECMAScript features) and regenerator-runtime/runtime (needed to use transpiled generator functions):

import "core-js/stable";
import "regenerator-runtime/runtime";

The @babel/polyfill module includes core-js and a custom regenerator runtime to emulate a full ES2015+ environment.

 

Now luckily for us, we're using the env preset which has a "useBuiltIns" option that when set to "usage" will practically apply the last optimization mentioned above where you only include the polyfills you need. With this new option the configuration changes like this:

{
  "presets": [
    [
      "@babel/env",
      {
        "targets": {
          "edge": "17",
          "firefox": "60",
          "chrome": "67",
          "safari": "11.1",
        },
        "useBuiltIns": "usage",---编译时候，会检查特征，只加载需要垫片
      }
    ]
  ]
}